Recently, as digital apparatuses represented by a personal computer are coming into wide use, projection-type imaging devices, which can project an image created by such a digital apparatus onto a screen, are entering the marketplace for the purpose of presentations and the like. In the projection-type imaging device, light from a light source is modulated by an image display element for displaying the image created by the digital apparatus, and the modulated light is magnified and projected by a projection optical system.
As the digital apparatuses are high-powered, images created by the digital apparatuses have high resolution. Thus, the projection-type imaging device is required to include an image display element having higher resolution. Further, a light source of the projection-type imaging device is required to have high luminance so as to realize a bright image on the projection screen.
The image display element of the projection-type imaging device is irradiated with light from the light source. Accordingly, by providing a light source having high luminance, the image display element is irradiated with more intense light. In the case of a transmission-type image display element, a temperature thereof rises significantly when the element is irradiated with intense light. Therefore, a high-luminance projection-type imaging device uses a reflection-type image display element. Even in the reflection-type image display element, a temperature thereof rises due to some light absorption when the element is irradiated with intense light. Accordingly, provision of a structure for forcibly cooling the image display element is required.
Further, by making the image display element have high resolution, the amount of heat that the image display element itself generates becomes great. Accordingly, in order to make a high-resolution and high-luminance projection-type imaging device, a structure for efficiently cooling the image display element is required.
Light from the light source is incident on the reflection-type image display element via a prism, and the light is reflected by a display surface of the reflection-type image display element and is again incident on the prism. Therefore, by providing the high-luminance projection-type imaging device, a temperature of the prism is increased. Thus, in order to make the high-luminance projection-type imaging device, a structure for efficiently cooling the prism is required.
FIG. 17 illustrates a structure for cooling an image display element 909 of a conventional projection-type imaging device. It should be noted that in FIG. 17, one of the three primary color components of light from a light source is focused, and therefore only the image display element 909 associated with the single primary color component is shown. The image display element 909 is a reflection-type image display element of a micromirror type. A plurality of micromirrors are placed on a display surface of the image display element 909. An angle of each micromirror is changed according to a control signal representing a prescribed image, so that an image is displayed on the display surface of the image display element 909.
The image display element 909 is joined to an electronic cooling element 912 via a holder 913. For example, the electronic cooling element 912 is formed of a semiconductor.
The electronic cooling element 912 is joined to a heat sink 914 and a cooling fan 915 for cooling the heat sink 914.
Light from a light source (not shown) is incident on the image display element 909 via a TIR prism 903 and a color separation/color combining prism 906. Light reflected by the image display element 909 is directed in a direction indicated by arrows 910a or 910b according to an angle of the micromirror. Light directed in the direction indicated by arrow 910a (light 910a) carries information on an image displayed on the display surface of the image display element 909. The light 910a is transmitted by the color separation/color combining prism 906 and the TIR prism 903, and thereafter the light 910a is projected onto a screen by a projection optical system (not shown).
The image display element 909 is cooled by the holder 913, the electronic cooling element 912, the heat sink 914 and the cooling fan 915.
Light directed in the direction indicated by arrow 910b is not used for projection onto the screen, and therefore is referred herein to as “extraneous light”. The extraneous light 910b is guided by the color separation/color combining prism 906 so as not to be incident on the TIR prism 903, and exits the color separation/color combining prism 906. If the extraneous light 910b is incident on the TIR prism 903, contrast in the image projected onto the screen is lowered, thereby deteriorating image quality. The extraneous light 910b is absorbed in a location (not shown) which is irradiated with the extraneous light 910b. 
Another example of the cooling structure of the reflection-type image display element is shown in Japanese Laid-Open Publication No. 10-319853. In the example shown in Japanese Laid-Open Publication No. 10-319853, an image display element is joined to a metal plate having high thermal conductivity, thereby increasing an area of image display element surfaces from which heat is radiated. The metal plate is cooled via natural convection.
Still another example of the cooling structure of the reflection-type image display element is shown in Japanese Laid-Open Publication No. 10-319379. In the example shown in Japanese Laid-Open Publication No. 10-319379, cooling fans are provided on the front and rear surfaces of an image display element so as to generate air convention for cooling. Japanese Laid-Open Publication No. 10-319379 also discloses a configuration in which heat of the image display element is conducted to radiation fins by a heat pipe and the radiation fins are cooled by a cooling fan.
In the conventional configuration shown in FIG. 17, a cooling fan 915 is provided to the image display element 909. In order to enhance the cooling efficiency of the image display element 909, the volume of air generated by the cooling fan 915 must be increased by increasing the number of rotations of the cooling fan 915 or by making the cooling fan 915 larger. However, in the configuration shown in FIG. 17, vibrations of the cooling fan 915, which are inevitably increased by an increase in air volume, are directly transferred to the micromirrors included in the image display element 909, deteriorating quality of an image displayed on the image display element 909. Consequently, quality of the image projected onto the screen is deteriorated. In particular, when the image display element 909 having high resolution is used, such deterioration in image quality is significant. The vibrations generated in the cooling fan 915 also reduce precision in the alignment of the image display element 909 with respect to the color separation/color combining prism 906, so that quality of an image projected onto the screen is deteriorated.
Therefore, according to the conventional configuration shown in FIG. 17, the image display element 909 cannot be efficiently cooled.
Further, in the configuration shown in FIG. 17, the color separation/color combining prism 906 is responsible for a function of guiding the extraneous light 910b so as not to be incident on the TIR prism 903. Thus, the color separation/color combining prism 906 cannot be made compact.
In the cooling structure disclosed in Japanese Laid-Open Publication No. 10-319853, no cooling fan for cooling the image display element is used, and thus no problem related to deterioration in image quality due to vibrations of such a fan as described above occurs. However, cooling by natural convection does not sufficiently cool the image display element and therefore cannot be applied to a high-resolution projection-type imaging device.
In the cooling structure disclosed in Japanese Laid-Open Publication No. 10-319379, the image display element is cooled by air. Since the thermal conductivity of air is low, efficient cooling cannot be provided. Further, in such a configuration where heat of the image display element is conducted to the radiation fins by the heat pipe, there is a difficulty in adjusting a position of the image display element with respect to the prism because of the high rigidity and heavy weight of the heat pipe, thereby reducing precision in alignment. In order to make the heat pipe less rigid and lighter, the heat pipe is required to be thinner. However, if the heat pipe is made thinner, the amount of heat that the thin heat pipe conducts in a unit of time becomes less, whereby the image display element cannot be efficiently cooled.
Further, none of the above-described publications related to conventional technologies refers to cooling of a prism itself (e.g., the color separation/color combining prism 906 shown in FIG. 17). There is a problem that when a temperature of the prism is increased, dimensions of the prism vary due to thermal expansion and precision in the optical system is reduced, thereby deteriorating quality of an image projected onto the screen.
The present invention is made in view of the above-described problems. An objective of the present invention is to provide a projection-type imaging device in which a prism can be compact. Another objective of the present invention is to provide a projection-type imaging device in which a reflection-type image display element can be efficiently cooled. Still another objective of the present invention is to provide a projection-type imaging device in which the prism can be efficiently cooled.